Nowadays, as a bonding wire (a wire used for connection in a wire bonding method) for connecting an electrode on a semiconductor device and an external terminal, there is mainly used a thin wire (bonding wire) having a wire diameter of about 13-50 μm. Wire bonding is generally performed through a thermocompression bonding method with the aid of ultrasonic waves. Here, there are used, for example, a general bonding device and a capillary jig allowing a bonding wire to be passed thereinside when performing bonding. Particularly, a wire front end is heated and melted through an arc heat input so as to allow a ball to be formed through a surface tension, followed by press-bonding this ball to an electrode of a semiconductor device heated in a range of 150-300° C., and then directly bonding the bonding wire to an external lead side through ultrasonic bonding.
In recent years, rapid diversifications have been observed in, for example, a semiconductor mounting structure, material and connection technology. For example, as for a mounting structure other than the existing QFP (Quad Flat Packaging) using a lead frame, there have been put to practical use new mounting methods including, for example: BGA (Ball Grid Array) using a substrate, a polyimide tape and the like; and CSP (Chip Scale Packaging). For this reason, there has been demanded a bonding wire with further improved properties including a loop property, a bondability, a mass productivity and the like.
A highly-pure 4N type gold (purity>99.99 mass %) has been mainly used as a material of a bonding wire. However, since gold is expensive, there has been desired a bonding wire made of an other kind of metal that is inexpensive in terms of material cost.
Patent document 1 discloses a bonding wire made of copper (referred to as “copper bonding wire” hereunder) that has been developed for the purpose of achieving, for example, a superior electric conductivity, an improved ball bondability and an improved wedge bondability at an inexpensive material cost. However, a copper bonding wire imposes problems including: a decrease in a bonding strength due to an oxidation on a wire surface; and a tendency of causing corrosion or the like to occur on the wire surface when encapsulated by resin. Those are also the reasons why the copper bonding wire has not yet been fully put to practical use in LSIs.
Materials to be bonded by a bonding wire mainly include, for example: a pure Al or an Al alloy as a wiring or an electrode on a silicon substrate. Here, the Al alloy is often an alloy of Al-1% Si, Al-0.5% Cu, Al-1% Si-0.5% Cu or the like. Even a Cu wiring for use in a micro wiring often includes an Al alloy or the like used on a surface thereof. Particularly, an Ag plating, a Pd plating or the like is performed on a lead frame, the Cu wiring is formed on a resin substrate, a tape or the like, and films of: a noble metal element such as gold or the like; and an alloy thereof are further formed on the Cu wiring. Here, it is required that a bondability and a bonding reliability of a bonding wire be improved to deal with the aforementioned various materials to be bonded.
As for the copper bonding wire, wire bonding is so performed that, for the purpose of controlling oxidation, a nitrogen gas or a nitrogen gas containing hydrogen by about 5% is sprayed on the wire front end when forming the ball thereon by melting the corresponding wire front end.
As is the case in the conventional gold bonding wire, the copper bonding wire is also required to satisfy reliability tests intended for semiconductors. A wire-bonded semiconductor, after being encapsulated by resin, is subjected to a reliability test for evaluating a practical use thereof in an accelerated manner. Typically, there are performed, for example: an HTS test (High Temperature Storage) featuring heating at a high temperature; a PCT (Pressure Cooker Test) which is a heating test performed in a high-humidity/temperature environment; and a HAST (Highly Accelerated Temperature and Humidity Stress Test). The PCT is also referred to as a saturated pressurized water vapor test whose general condition includes heating at a temperature of 121° C. and a relative humidity (RH) of 100% with no electrical bias being applied. Meanwhile, the HAST is also referred to as an unsaturated pressurized water vapor test whose general condition includes heating at a temperature of 130° C. and a relative humidity (RH) of 85% with an electrical bias being applied.
As compared to the conventional gold bonding wire, a few reports have been made on a bonding reliability of a bonded portion formed of a copper bonding wire and an aluminum or lead electrode. Patent document 3 and Non-patent document 1, for example, report on a bonding reliability of a Cu/Al bonded portion. It is known that intermetallic compounds in the Cu/Al bonded portion grow significantly more slowly than those in an Au/Al bonded portion. Therefore, it is assumed that the copper bonding wire, when heated at a high temperature, exhibits a more favorable bonding reliability than that of the gold bonding wire, due to slow growth rates of the intermetallic compounds in the Cu/Al bonded portion. Since there are no past records indicating that the copper bonding wire has ever been used in LSIs, no full study has ever been carried out on, for example, a reliability of the copper bonding wire being used, thus leaving a standard for a reliability evaluation, a life or the like unclear.